1. Field of the Invention
The present invention relates to the technology field of OLEDs, and more particularly to a light-emitting material capable of facilitating an OLED emit deep blue light.
2. Description of the Prior Art
It is well known that organic light emitting diode (OLED) was initially invented and proposed by Eastman Kodak Company through a vacuum evaporation method. Tang and VanSlyke of Kodak Company deposited an electron transport material such as Alq3 on a transparent indium tin oxide (abbreviated as ITO) glass formed with an organic layer of aromatic diamine thereon, and subsequently completed the fabrication of an organic electroluminescent (EL) device after a metal electrode is vapor-deposited onto the Alq3 layer. The organic EL device has become a new generation lighting device or display because of high brightness, fast response speed, light weight, compactness, true color, no difference in viewing angles, without using any LCD backlight plates, and low power consumption.
Recently, some interlayers such as electron transport layer and hole transport layer are added into the OLEDs for increasing the current efficiency and power efficiency of the OLEDs. For example, the organic light emitting diode (OLED) 1′ shown as FIG. 1 is consisted of: a cathode 11′, an electron injection layer 13′, a light emitting layer 14′, a hole transport layer 16′, and an anode 18′.
In fundamental principle, OLED 1′ would emit light after a differential voltage is applied between the anode 18′ and the cathode 11′, wherein the light is emitted as the excitons produced by the combination of electrons and holes diffuse into the light emitting layer 14′. According to theoretical speculation, the ration of the excitons with singlet excited state and the excitons with triplet excited state is 3:1. So that, when a small molecular fluorescent material is used as the light-emitting layer 14′ of the OLED 1′, there are about 25% excitons being used in emitting light, and the rest of 75% excitons with triplet excited state are lost through non-luminescence mechanism. For this reason, the general fluorescent material performs a maximum quantum yield of 25% in limit.
Fluorescent materials are broadly studied in the initial development of OLEDs, and the OLED having first generation blue fluorescent material performs a maximum EQE (External Quantum Efficiency) of 5%. Although the first generation blue material shows the advantages of high thermal stability and reliable material life, the OLED having the first generation blue material can merely emit a baby blue light, but cannot emit a pure blue light and/or a deep blue light. Typical blue fluorescent material is made of distyrylarylene (DSA) derivative and proposed by Idemitsu Kosan Company in Japan.
Flrpic (Bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III)) is an exemplary material of second generation blue phosphor material with the CIE coordinates of (0.17, 0.34). Moreover, by substituting the Pyrazoly Borate group for the secondary group of Picolinate in Flrpic, a novel blue phosphor material is manufactured by Professor Thompson, wherein the novel blue phosphor material is named FIr6 (iridium(III)bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyraolyl)borate). The OLED having FIr6 as the light-emitting layer 14′ is able to emit a blue light with CIE coordinates of (0.16, 0.26) and performs the power efficiency of 13.91 lm/w. In spite of that, The OLED 1′ having FIr6 still cannot emit a pure blue light and/or a deep blue light. Herein, the standard CIE coordinates of deep blue made by NTSC (National Television System Committee) is (0.14, 0.08).
In recent years, research papers proposed a variety of host light-emitting material made of carbazole derivatives, for example, CDBP (4,4′-bis(9-carbazolyl)-2,2-dimethyl-biphenyl). Experiment results have been proved that, OLED's EQE can reach 10.4% when CDBP and Flrpic are respectively used as the host light-emitting material and the guest light-emitting material. However, the aforesaid materials cannot be applied in the fabrication of commercial OLED due to their unreliable material life.
Accordingly, in view of the conventional blue light-emitting materials cannot used for manufacturing OLEDs having the advantages of high EQE, long life time and including the standard CIE coordinates of deep blue, the inventor of the present application has made great efforts to make inventive research thereon and eventually provided a novel light-emitting material for organic light emitting devices (OLEDs).